Table of contents

When a tank of water, which is usually used to simulate human tissue, is exposed to a broad beam of X-rays the dose distribution in the water, in any plane, can be described by a family of isodose curves. The device described in this article draws such curves by recording the path of a small ionization chamber, which automatically follows preselected isodose lines, in the water. The design factors which determine the accuracy of the results are discussed.

This paper describes an electropolishing technique for the preparation of thin foils for examination by transmission electron microscopy. The technique utilizes a special holder constructed of polytetrafluoroethylene and is capable of producing consistently good foils in a few minutes.

An absolute method of making simultaneous measurements of the transmission and reflection coefficients of semiconductors and insulators to a precision of better than 1% has been developed for use in the visible and infra-red regions of the spectrum. The reduction of these measurements to the optical constants by a technique taking full account of the finite spectral bandwidth of the light is also described.

The minimum power or energy detectable by calorimeters is considered from the point of view of thermal fluctuations. Essential noise, detectivity and figure of merit are defined for power- or energy-measuring calorimeters. The figures of merit of some microcalorimeters are compared. Design considerations for thermopile instruments are analysed. A calorimeter with a single container built on these lines is described and its performance discussed. Its essential features are: heat capacity 10 J degC^-1; time constant 180 sec; noise-equivalent power 10^-7 w; accuracy to within 0.2%; range 10^-1-10^-7 w.

A theoretical analysis has been made of the `multipactor' dagger or `dynamic secondary emission' process as used in an image intensifier tube described by the authors in 1962. Electron multiplication equivalent to several stages of secondary emission is achieved by repeated use of a single emitting surface serving both as photocathode and as dynodes. The equations of motion of the electrons have been solved by computer and the results are presented as universal curves which may be used to predict the performance of all tubes of the class considered. Using reasonable dimensions and voltages it is not difficult to obtain secondary emission yields of six per cycle for any convenient number of cycles. Up to 80% of the photoelectrons emitted during the first cycle will be caught up in the multiplying system and there is no loss of secondaries. Instability is avoided by stopping the process after several cycles and starting again with new photoelectrons.

Sodium and sodium amalgam have been found to act as non-polarizing electrodes for soda glasses, and an electrode assembly is described which is suitable for accurate dielectric measurements. The elimination of edge relaxation effects by means of a guard ring was found to be essential. Comparative measurements with silver electrodes suggest that these do not act as simple blocking electrodes but that a diffusion layer arises of much greater thickness than the Debye length.

A circuit is described which will sweep a magnetic field by electronic means about a stabilized value. Sweep amplitudes from 1 mA to 3 A corresponding to typical field changes of 1 Oe to 2500 Oe in periods from 1 sec to 1 hour are obtainable. The application of the system to conventional electromagnets is discussed.

A bulbless form of McLeod gauge is described which has been used to measure pressures within the range 10^-2 to 1250 torr. The pressure scale can be calculated when the length of the measuring tube is known. Procedures for this are discussed, and a table of values given for a particular gauge. Means of setting the mercury level in the gauge when reading pressures below and above atmospheric are indicated.

Complete details are given of a 1 kw precision magnet current supply. Series transistors are used as the regulating element, controlled by a solid state chopper amplifier. The nature of the feedback system permits use of the supply with magnets of any impedance and with large amplitude (500 v r.m.s. 50 c/s) external magnet modulation. D.C. drift is less than 10 parts per million per hour and noise in the 0.1-1 c/s band less than 1 part per million.

A temperature probe for measuring local temperatures in a flowing polymer melt is described. A special construction is given for the case when the wall of the processing machine (e.g. an extruder) is at a lower temperature than the melt. The instrument proper is contained in a stainless-steel tube which is inserted through a hole in the wall of the processing machine.

The instrument consists of two thermocouples, their respective positions being near the end of the tube and a certain distance from this end, i.e. closer to the wall of the machine. In addition, that part of the probe which is between the wall and the thermocouples can be heated electrically for compensating the heat loss by conduction to the wall. This compensation is achieved when the two thermocouples are brought to the same temperature.

A theoretical consideration shows that by varying the temperatures of the thermocouples with the aid of the additional heating, the heat transfer coefficient can be determined for transfer from the steadily flowing melt to the temperature probe. For practical purposes the instrument described may seem somewhat difficult to handle; for more fundamental investigations, however, it proved indispensable.

A simple microfurnace is described which has been used on a standard oscillation camera for high temperature single-crystal x-ray studies. The furnace incorporates an element of fine platinum-13% rhodium wire. Examples of x-ray photographs obtained with the high temperature x-ray camera are presented.

A small gas-sampling pump is described which is operated by solenoids driven by transistors. This arrangement has been used to satisfy the requirements of explosion-proof operation from a low-voltage supply, small size and weight, and reliability.

The fitting of variable resistances to the heating circuits of two commercial thermostat-stirring units is described. The output of the heater can then be matched to the loss of heat from the bath, giving greatly improved control. A switch to short out the variable resistance gives in effect a boost heater which is switched off when the desired temperature is reached.

A simple and compact device for oscillating and rotating an ingot during a back-reflection x-ray exposure, using a single motor, is described. The device can be used to take a spectrum type pattern at liquid air temperature.

A repetitive light source with reasonably fast rise time and high intensity was required in order to transmit a timing pulse into the high voltage terminal of a d.c. accelerator. A suitable source has been developed which provides all the required features using a special type of stroboscopic tube. The main advantage is that the `jitter' or variation in time delay is extremely small and the system could be developed for nanosecond timing applications. The light source may also be used as a high speed stroboscope.

A cryostat has been constructed from available laboratory apparatus to provide steady temperatures between -20 and -140°C.

Temperature controllers similar to the one used are commonly available in laboratories and need no modification to control sub-ambient temperatures. The temperature stability was of the order ±1 degC at -60°C but this could probably be improved if necessary.

An automatic liquid level sensing device for liquid nitrogen has been devised. The alterations necessary to convert a commercially available refrigeration temperature controller for proper operation are described.

The liquid nitrogen cooled cold traps used on vacuum systems need replenishing fairly frequently, especially if the liquid nitrogen is to be maintained at a a constant level. A simple bridge circuit is described which performs this task automatically. It is robust and reliable, has only one electronic valve and does not require a d.c. power source.

Corrections to 1963 J. Sci. Instrum.40 340.